1. Field of the Invention
The present invention relates to a device and a method for evaluating amplification characteristics of optical amplifiers, particularly to a device and a method for evaluating gains and noise figures of optical amplifiers which carry out wavelength multiplex amplification.
2. Background Art
Semiconductor optical amplifiers are known as a representative example of an optical amplifier which is presently studied. The semiconductor optical amplifier is made up of a semiconductor laser oscillator which has a laser light output surface covered with a special anti-reflection coat to suppress the oscillation.
As an another example, there is an optical fiber amplifier which is made up of an optical fiber in which a rare earth element such as an erbium and an praseodymium is doped.
In the optical fiber amplifier, when an excitation light is inputted in the optical fiber in which a rare earth element is doped, a population inversion is formed among different energy levels of the rare earth element. If a signal light is inputted to the optical fiber and the signal light has an energy corresponding to an energy difference between energy levels of the rare earth element which is in a population inversion state, a stimulated emission is caused in the optical fiber by the signal light and an amplified signal light is obtained by the stimulated emission. In this amplification, an ASE (Amplified Spontaneous Emission) light having spectrums in a broad band is outputted from the optical fiber amplifier together with the amplified signal light.
Next, the description will be given with respect to noises included in output lights of optical amplifiers.
In an optical amplifier, an average photon number &lt;N.sub.0 &gt; of an output light of the optical amplifier is defined as follows: EQU &lt;N.sub.0 &gt;=G&lt;Ni&gt;+(G-1)mt n.sub.sp .DELTA..nu. (1)
Furthermore, the variance .SIGMA..sub.0 of the photon number is defined as follows: ##EQU1##
In the above equations, G is a gain of the optical amplifier, and &lt;N.sub.i &gt; is the average photon number of an input light of the optical amplifier, and n.sub.sp is a population inversion parameter, and .DELTA. .nu. is a light frequency band width of an ASE light included in an output light of the optical amplifier. Furthermore, mt is a transversal mode number of ASE light. In the case of polarization independent optical amplifiers, mt is 2. In the case of semiconductor laser amplifiers, mt is 1.
In the equation (1), the first term defines an amplified signal light included in the output light and the second term defines the ASE light included in the output light. In the equation (2), the first term defines a shot noise of the amplified signal light, and the second term defines a shot noise of the ASE light, and the third term defines a beat noise which is caused between the signal light and the ASE light, and the fourth term defines a beat noise which is caused by different spectrums of the ASE light.
The noise figure F of the optical amplifier is defined by a ratio of a S/N ratio (S/N).sub.in of the input signal of the optical amplifier and a S/N ratio (S/N).sub.out of the output signal as follows: EQU F=(S/N).sub.in /(S/N).sub.out ( 3)
In the equation (3), each S/N ratio is determined as follows: EQU S/N=e.sup.2 &lt;N.sub.sig &gt;/(2e.sup.2 .SIGMA..sup.2 Be) (4)
In the above equation (4), e is an electric charge of an electron, and Be is an equivalent noise band width of an O/E (Optical/Electrical) converter which receives the input or output light of the optical amplifier, and &lt;N.sub.sig &gt; is an average photon number of the input light, and .SIGMA..sup.2 is a variance of the photon number.
The ratio (S/N).sub.in of the equation (3) can be obtained by calculating &lt;N.sub.0 &gt; of the equation (1) at G=1 and .SIGMA..sup.2 of the equation (2) at G=1, and by calculating the S/N of the equation (4) using the &lt;N.sub.0 &gt; and .SIGMA..sub.0.sup.2 thus calculated instead of the &lt;N.sub.sig &gt; and the .SIGMA..sup.2. On the other hand, the ratio (S/N).sub.out can be obtained by entering the &lt;N.sub.0 &gt; of the equation (1) and the .SIGMA..sub.0.sup.2 of the equation (2) into the equation (4) instead of the &lt;N.sub.sig &gt; and the .SIGMA..sup.2.
The noise figure F of the optical amplifier is obtained by entering the (S/N).sub.in and the (S/N).sub.out thus obtained into the equation (3) as follows: ##EQU2##
In the above equation (5), the population inversion parameter n.sub.sp and the average input photon number &lt;N.sub.i &gt; are defined as follows: EQU n.sub.sp =P.sub.ASE /(h.nu.(G-1)mt.DELTA..nu.) (6) EQU &lt;N.sub.i &gt;=P.sub.in /(h.nu.) (7)
In the above equations, P.sub.ASE is a total light power of the whole ASE light, and h is a Planck's constant, and .nu. is a frequency of the signal light, and P.sub.in is a light power of the input signal light.
In the case of an optical fiber amplifier, the number mt of the transversal modes of the ASE light is 2. Therefore, the equation (5) is rewritten as follows: ##EQU3##
In the above equation (8), P.sub.ASES is a light power of the ASE light at the light frequency of the signal light and .DELTA..nu.s is a light frequency band of a light receiver for receiving the output light of the optical amplifier and for determining the P.sub.ASES.
Next, the description will be given with respect to a conventional technique for evaluating an optical amplifier which amplifies a signal light and outputs an amplified signal light with a noise as described above.
FIG. 12A is a block diagram showing the configuration of a conventional device for evaluating a noise figure of an optical amplifier which amplifies a signal light having a single wavelength or a multiplex wavelength signal light.
In FIG. 12A, light sources 101.sub.-1, 101.sub.-2, . . . , 101.sub.-n respectively generate signal lights respectively having wavelengths .lambda..sub.-1, .lambda..sub.-2, . . . , .lambda..sub.-n. The signal lights generated by the light sources 101.sub.-1, 101.sub.-2, . . . , 101.sub.-n are mixed by a light mixer 102 to generate an wavelength multiplex signal light. The wavelength multiplex signal light thus generated is then supplied to an optical amplifier 103, the characteristics of which are to be determined. FIG. 12B shows an example of light spectrums of this wavelength multiplex signal light supplied to the optical amplifier 103. The wavelength multiplex signal light is amplified by the optical amplifier 103 and the output light of the optical amplifier 103 is analyzed by an optical spectrum analyzer 104.
In the conventional art, a noise figure of the optical amplifier is determined based on a spectrum distribution of spectrums of the output light which is analyzed by the spectrum analyzer 104.
FIG. 12C shows light spectrum of the output light of the optical amplifier 103. As shown in FIG. 12C, the output light of the optical amplifier 103 includes a broad band ASE light and the ASE light has a very complex spectrum distribution. Thus, it is difficult to determine the second item and the fourth item of the equation (8). Therefore, the noise figure F of the optical amplifier 103 is approximately calculated taking the shot noise of the signal light and the beat noise caused due to the signal light and the ASE light in account and ignoring the second item and the fourth item of the equation (8). The equation for approximately calculating the noise figure F is as follows: EQU F.apprxeq.(1/G)+(P.sub.ASE /(h.nu.G.DELTA..nu.s)) (9)
If the optical amplifier 103 has a gain G and the output light of the optical amplifier 103 has a light power P.sub.out, the gain G is defined as follows: EQU G=(P.sub.out -P.sub.ASE)/P.sub.in ( 10)
In order to determine noise figures of the wavelength multiplex amplification, it is necessary to determine the parameters defined in the equations (9) and (10) for the wavelengths of the signal lights which constitute the wavelength multiplex signal light. The noise figure F can be calculated wavelength by wavelength using the parameters thus determined.
In the conventional method, however, the noise figure F of the wavelength multiplex amplification is approximated by taking only the shot noise of the signal light and the beat noise caused due to the signal light and the ASE light. Therefore, the method is not to evaluate the optical amplifier which actually operates as a wavelength multiplex amplifier.
Furthermore, the conventional method for evaluating noise figures of wavelength multiplex amplification requires to determine powers of the ASE light at the wavelengths of the signal lights. However, it is difficult to determine the powers.
The description will be given with respect to this problem.
FIG. 13A shows an example of light powers P.sub.in-1, P.sub.in-2, . . , P.sub.in-n of spectrums of an wavelength multiplex signal light inputted to the optical amplifier 103 which respectively correspond to wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n. FIG. 13B shows an example of light powers P.sub.out-1, P.sub.out-2, . . . , P.sub.out-n of spectrums of amplified signal lights outputted from the optical amplifier 103 which respectively correspond to the wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n. These light powers of the spectrums may be easily measured by an optical spectrum analyzer.
However, it is impossible to directly determine the light powers P.sub.ASES-1, P.sub.ASES-2, . . . , P.sub.ASES-n of the ASE light corresponding to the wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n because the ASE light is outputted from the optical amplifier together with the amplified signal lights and the light powers of the ASE light at the wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n are buried in the light powers the the amplified signal lights.
In order to obtain the light powers P.sub.ASES-1, P.sub.ASES-2, . . . , P.sub.ASES-n of the ASE light, it is necessary to determine the light powers of the output light of the optical amplifier at wavelengths neighboring the wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n and to interpolate the light powers P.sub.ASES-1, P.sub.ASES-2, . . . , P.sub.ASES-n based on the light powers at the neighboring wavelengths as shown in FIG. 14.
However, such a manual operation is difficult and labor. Furthermore, the analysis by the spectrum analyzer is influenced by a stray light. Therefore, it is difficult to accurately interpolate the light powers P.sub.ASES-1, P.sub.ASES-2, . . . , P.sub.ASES-n.
There is a method to suppress the input signal light of the optical amplifier by using a light polarization controller and a polarizer in order to improve the accuracy of the interpolation of the ASE light powers P.sub.ASE-1, P.sub.ASE-2, . . . , P.sub.ASE-n or to reduce the influence due to a stray light. However, it is difficult to accurately determine the ASE light powers even if the method is used.
Furthermore, the influence of the stray light is increased when the steps between the wavelengths of the multiplexed signal lights are very short or less than the resolution of the optical spectrum analyzer as shown in FIG. 15. Thus, the measurement error of the ASE light powers is increased due to the influence. Therefore, it is very difficult to accurately and smoothly determine the wavelength multiplex amplification characteristics of the optical amplifier.